1. Field of the Invention
This invention relates generally to Fabry-Perot filters. More specifically, the invention relates to tunable Fabry-Perot filters for use in wavelength division multiplexing and dense wavelength division multiplexing which have micro-electro-mechanical systems machined parts to tune the filter.
2. Description of the Related Art
In present-day long-haul fiberoptics, it is typically necessary to multiplex and demultiplex a single channel of dense wavelength division multiplexed systems and to select desired frequency bands. To perform these tasks, fixed filters and bulky, tunable filters have been used. Both of these types of prior art filters are largely ineffective since they cannot provide the desired bandwidth resolution needed for modern fiberoptic communication systems, and each of these systems moreover tends to become easily misaligned, thereby degrading the performance of the optical systems in which they are found.
Several approaches have been employed in the past to provide variable optical attenuators (VOAs) for use in broadband, optical, dense wavelength division multiplexing (DWDM) amplifiers and for controlling the input power level to the amplification stages of such long-haul systems. A VOA usually includes an input fiber, an output fiber, a lens to focus light, and a mirror which can be adjusted to change the amount of light traversing through the lens from the input fiber as the light makes its way back to the output fiber. By varying the angle of the mirror, the amount of light received by the output fiber is varied, thereby changing the transmission of the signal. However, VOAs are not adaptable for tuning the signal or for selecting an appropriate frequency band for the fiberoptic system.
Fabry-Perot filters or interferometers, as these terms are used interchangeably throughout, are well known devices that function both as optical spectrum analyzers, channel demultiplexers, and frequency bandpass filters in existing large scale optical systems. However, due to their relatively large and bulky size, as well as the need to provide mirror adjustments, Fabry-Perot interferometers have not made their way into modern, long-haul fiberoptic communication systems. A Fabry-Perot interferometer generally includes an input and output light conduit, for example a fiber, a lens to focus the input light on a stationary mirror, a lens to focus the output light on the output conduit, and a second mirror which is movable so that the length between the stationary mirror and the fixed mirror, sometimes called the cavity or xe2x80x9cetalonxe2x80x9d, can be varied, to thereby vary the optical path length of the light traversing through the cavity. By varying the optical path length, the output light can be multiplexed, demultiplexed, filtered, or band-passed depending on the particular application in which the Fabry-Perot interferometer will be used.
Neither prior art VOAs nor Fabry-Perot interferometers have been effectively implemented in fiberoptic communication systems since both are rather bulky and are difficult to control when reduced in size to fit such systems. Indeed, attempts have been made in the past to miniaturize VOAs in order to package such devices on integrated circuits and place them in fiberoptic communication systems. However, since VOAs are very sensitive and a slight aberration in mirror angle virtually destroys their effectiveness, VOAs cannot function well in long-haul DWDM fiberoptic systems. Moreover, there has not been developed for miniaturized Fabry-Perot filters an effective means for controlling the variable mirror to attain an accurate way of adjusting the cavity length.
There is, accordingly, a long-felt but unresolved need in the art for miniature optical devices that can be used in long-haul fiberoptic communication systems that will efficiently provide DWDM of channels. Moreover, these devices should be simple to fabricate and control. Such needs have not heretofore been fulfilled in the art.
The aforementioned long-felt needs are met, and problems solved, by Fabry-Perot filters provided in accordance with the present invention. The inventive Fabry-Perot filters are particularly useful for wavelength division multiplexing (WDM) and DWDM of optical channels in long-haul fiberoptic communication systems, which are today becoming ubiquitous for large networks and telecommunication systems. A preferred embodiment of the Fabry-Perot filters disclosed and claimed herein comprises a variable length cavity through which electromagnetic energy of a plurality of wavelength channels will traverse in two opposite directions, and a fixed mirror at one end of the cavity. A variable mirror preferably disposed at the other end of the variable length cavity can be moved transversely in the direction of the length of the cavity to vary the cavity""s length. Advantageously, a micro-electro-mechanical systems (MEMS) machined part is disposed in contact with the variable mirror for selectively applying force to predeterminedly move the variable mirror along the length of the cavity.
The Fabry-Perot filters of the present invention are excellent devices for multiplexing and demultiplexing dense channels of optical data in modern telecommunication systems. The inventive filters are easily fabricated on a silicon chip that employs MEMS technology and so may be economically mass-produced. The Fabry-Perot filter silicon chips in accordance with the present invention will have low insertion loss, low polarization dependent loss, and a flat response over the desired bands. Furthermore, such Fabry-Perot filters require only a low DC drive voltage to operate. With the use of MEMS technology to produce a mechanical control device for the variable mirror of the inventive Fabry-Perot filters, an efficient and easily controlled filter is obtained. Such results have not heretofore been achieved in the art.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.